Participation of a heterolytic path in the photochemistry of chlorobenzene

Article abstract of DOI:10.1016/j.jphotochem.2010.01.005

The photochemistry of chlorobenzene in different media has been studied in the presence of oxygen as well as of allyltrimethylsilane and benzene. These are selective traps of the phenyl radical and of the phenyl cation, respectively. Photo-homolysis is a primary process, under most conditions the main processes. However, in an acidic alcohol such as 2,2,2-trifluoroethanol (TFE), this is substituted by photoheterolysis, though with a lower efficiency.

Full text of DOI:10.1016/j.jphotochem.2010.01.005

Journal of Photochemistry and Photobiology A: Chemistry 210 (2010) 140–144
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Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Participation of a heterolytic path in the photochemistry of chlorobenzene
Simone Lazzaroni, Stefano Protti, Maurizio Fagnoni, Angelo Albini∗
Dep. Organic Chemistry, University of Pavia, Via Taramelli 10, 27100 Pavia, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
The photochemistry of chlorobenzene in different media has been studied in the presence of oxygen
as well as of allyltrimethylsilane and benzene. These are selective traps of the phenyl radical and of
the phenyl cation, respectively. Photo-homolysis is a primary process, under most conditions the main
processes. However, in an acidic alcohol such as 2,2,2-trifluoroethanol (TFE), this is substituted by pho-
toheterolysis, though with a lower efficiency.
Received 10 October 2009
Received in revised form
31 December 2009
Accepted 8 January 2010
Available online 18 January 2010
© 2010 Elsevier B.V. All rights reserved.
Keywords:
Photoheterolysis
Dehalogenation
Aryl halides
Chlorobenzene
Phenyl cation
1. Introduction
While homolytic fragmentation appears to be well established,
its heterolytic counterpart has not often been favored, although it
Haloaromatics are among the most ubiquitous and persistent
pollutants [1–5] and their fate in the environment is a matter of
concern. Photochemistry appears to contribute significantly to abi-
otic degradation paths and this has favored research in the field.
In particular, the photochemistry of the simplest haloaromatic,
chlorobenzene (1) has been repeatedly investigated during the last
decades [6–12] and indeed has originated intriguing mechanistic
issues.
To summarize, the irradiation of 1 under various conditions
causes mainly two processes, reduction to benzene and, in nucle-
ophilic solvents (e.g. alcohols), solvolysis. Notice that both of these
involve the labilization of the C–Cl bond, not an expected reac-
tion from the initially formed ␲␲* states. This point has been
investigated in depth by experimental [13–16] and computational
studies [17–21]. In this way, the viability of homolytic fragmen-
tation (Scheme 1, path a), finally resulting in overall reductive
dehalogenation via the phenyl radical, has been demonstrated [12].
As for solvolysis, various hypotheses (highlighted in Scheme 1) have
been considered. These differ for the key intermediate involved, viz.
a radical cation formed by photoionization (path b), a polar exciplex
(see below), or the cation resulting from photoheterolysis (path
c) [2,12,22–25]. The competition between the above mechanisms
may then change according to conditions.
has been early proposed [26]. On the other hand, such a mechanism
(path c) has been shown to be favored for electron-donating sub-
stituted chlorobenzenes [27,28]. Indeed, the triplet state of these
compounds cleaves, at least in polar solvents, to give triplet phenyl
cations, which are synthetically useful intermediates. One may
wonder whether such a path has some role also with parent 1 under
suitable conditions.
In view of the above, we decided to study the photochemical
behaviour of chlorobenzene in various solvents as well as in the
presence of diagnostic traps in an attempt to distinguish the con-
tributions of different mechanisms to the photochemistry of this
important molecule and whether selectivity could be induced.
2. Experimental
Photolyses were carried out on 15 mL samples of 5 × 10−2 M
solutions of chlorobenzene (1) in a 1 cm optical path quartz tube.
After flushing with nitrogen or oxygen, the tube was stoppered
and irradiated by means of four 15 W low pressure mercury lamps
(emission at 254 nm) for 4 h. The irradiated solution was examined
by gas chromatography and products 2–5 (for a previous charac-
terization of ether 3 see Ref. [29]), as well as of bicyclohexyl and
phenylcyclohexane were identified by comparison of the GC/MS
data with authentic samples. Their amount was assessed on the
basis of calibration curves with authentic samples. Quantum yields
were measured with the same set up, by using 3 mL samples of
1 × 10−2 M solutions. The light flux was measured by ferrioxalate
actinometry.
∗
Corresponding author. Tel.: +39 0382 987316; fax: +39 0382 987323.
E-mail address: angelo.albini@unipv.it (A. Albini).
1010-6030/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2010.01.005

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141
Table 2
Products formed by irradiation of chlorobenzene (1, 5 × 10−2 M) in the presence of
allyltrimethylsilane (1 M).
.
Solvent
Substitution, %
Arylation, %
C6H12
MeOH
CF3CH2OH
CF3CH2OH/O2
4 (7)
4 (55)
3 (8)
3 (tr)
4 (8), (43)a
4 (tr), (53)a
a
In the presence of Et3N, 5 × 10−2 M.
Scheme 1. Photochemistry of 1.
Fluorescence spectra were measured by means of a LS-55
PerkinElmer spectrofluorimeter and the fluorescence intensity
(area of the corrected spectrum) compared with that in cyclohex-
ane (for which the quantum yield has been determined as 0.007)
[30]. The difference was within 20%.
Flash photolysis experiments were carried out by means of an
Applied Photophysics instrument fitted with a Lumonics Nd YAG
laser operating on the fourth harmonic (266 nm) and delivering
about 2 mJ per pulse.
The reaction was first examined in some neat solvents (see
Table 1). In cyclohexane reduction was accompanied by the for-
mation of bicyclohexyl and phenylcyclohexane as radical coupling
products. In alcohols, substitution of the carbon–chloro with a
carbon–oxygen bond atom took place to some degree, giving
anisole (2) in methanol and 2,2,2-trifluoroethoxybenzene (3) in tri-
fluoroethanol. The reactions were then repeated in the presence
of triethylamine (Et3N) equimolecular with 1, in order to buffer
hydrogen chloride potentially formed in the reaction. As a matter
of fact, this addition had virtually no effect on the photoreaction in
cyclohexane. On the other hand, it increased the consumption of
1 in methanol (as shown in Table 1), where however the amount
of anisole formed did not significantly change. In the case of TFE,
the consumption of 1 remained unchanged while the yield of ether
3 considerably increased. Since the substitution pathway was the
most significant in this case, the effect of oxygen saturation was
tested, finding that the consumption of 1 increased, while the for-
mation of the ether was strongly inhibited.
Further irradiations were carried out in the presence of ATMS
(1 M, see Table 2). Under these conditions, a small amount of allyl-
benzene (4) was formed in cyclohexane and much more (around
50%) in methanol (see Table 2). The yield of 4 was lower in TFE (8%),
where a comparable amount of ether 3 was formed. However, the
presence of triethylamine levelled the yield of alkene arylation to
ca. 50% also in this instance. In alcohols, formation of 4 was accom-
panied by partial or complete quenching of the ethers formation.
Furthermore, oxygen saturation had only little effect on the yield
of 4 in the presence of Et3N, whereas in the absence of the amine
both the ether and allylbenzene were no more formed.
3. Results
3.1. Photochemistry
In this work, the photochemistry of 1 has been explored in
neat solvent as well as in the presence of typical ␲ nucleophiles,
allyltrimethylsilane (ATMS) and benzene, known as specific traps
of triplet phenyl cations. The effect of oxygen as a trap of phenyl rad-
icals has been also observed and compared with previous results
reported in literature [12,31]. The experiments were carried out
in media of different polarity and nucleophilicity, cyclohexane,
methanol and 2,2,2-trifluoroethanol (TFE). Since one of the aims
of the work was to detect a potential selectivity, the experiments
were carried out at a relatively high starting concentration of 1
(5 × 10−2 M) and high conversion (>50%), so that it could be judged
whether the results had some preparative significance. The effect of
experimental parameters was assessed by comparing the results of
irradiations at a fixed time. Under all conditions, reductive dehalo-
genation to benzene occurred to a large extent. The other products
formed and the corresponding yields with respect to consumed 1
are reported in Tables 1–3.
Irradiations in the presence of 1 M benzene led, except for the
irradiations in cyclohexane, to the formation of biphenyl, in a mod-
est amount (12–19%) and again at the expenses of the ethers (see
Table 3).
Table 1
Products formed by irradiation of chlorobenzene (1), 5 × 10−2 M.
Table 3
Products formed by irradiation of chlorobenzene (1) in the presence of benzene
(1 M).
.
Solvent
ε
Substitution, %
Neat solvent
0.05 M Et3N
.
a
C6H12
2.0
Solvent
Substitution, %
Arylation, %
MeOH
CF3CH2OH
CF3CH2OH/O2
32.6
26.5
2 (20)
3 (31)
3 (tr)
2 (12)b
3 (42)
3 (10)
C6H12
MeOH
CF3CH2OH
None
5 (12)
5 (19)
2 (6)a
3 (19)a
a
Bicyclohexyl and phenylcyclohexane formed.
Under this condition the photochemical consumption of 1 increases by 84%.
b
a
No significant change in the presence of Et3N, 5 × 10−2 M.

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S. Lazzaroni et al. / Journal of Photochemistry and Photobiology A: Chemistry 210 (2010) 140–144
Fig. 1. (left) (a) Profile of the transient absorption at 320 nm observed upon flashing a nitrogen flushed solution 5 × 10−3 M of 1 in cyclohexane; (b) the same in an oxygen
flushed solution. (right) 2 (a) as before; (b) the same in the presence of 10−2 M ATMS.
Table 4
Quantum yield of reaction [˚R, ˚R (O2) is the value in oxygen saturated solution]
of chlorobenzene (1), as well as triplet lifetime (ꢀT) and rate of reaction of the latter
intermediate with ATMS (kq).
Solvent
˚
R
˚
R
(O2)
ꢀT, s
kq, mol−1 s−1
C6H12
CF3CH2OH
.27
.065
.065
.033
1.1 × 10−6
3.0 × 10−7
1 × 108
2 × 108
3.2. Mechanistic investigations
A series of photophysical experiments were carried out with the
aim of obtaining further evidence on the mechanisms involved in
the photodegradation of 1 (see Table 4). The fluorescence of 1 was
found to be little affected by the medium characteristics, with the
intensity varying by no more than 20% in methanol and trifluo-
roethanol in comparison to cyclohexane (for which the value ˚F
0.007 has been determined) [30].
The quantum yield of reaction was then measured on 1 × 10−2 M
solutions at a conversion lower than 20% and was found to be larger
in cyclohexane than in trifluoroethanol. However, in the first case
Fig. 2. Transient absorption observed 1 ␮s after flashing a nitrogen-flushed solution
5 × 10−3 M of 1 in TFE. Inset (a) Profile of the above transient absorption at 320 nm;
(b) the same in an oxygen flushed solution.
˚
R
was reduced to 1/4 upon saturation of the solution by oxygen,
is that the fluorescence little changes in going from cyclohexane
to ionic media while both reaction quantum yield and product
distribution undergo marked changes (e.g. ˚R is reduced to 1/4
and the triplet lifetime to 1/3 in going to trifluoroethanol). This
supports the notion that the singlet is scarcely affected by the
solvent polarity, the main process remaining ISC (˚ISC = 0.7) [32],
and that the photoreactions essentially proceed from the triplet
state.
in the latter to 1/2.
A flash photolysis investigation in cyclohexane revealed a con-
spicuous transient at 300, 320 nm. This was fully quenched in
oxygen-equilibrated solution, where it gave place to a different
transient with a weak absorption at 300–330 and 470 nm with a
rise time of ca. 100 ns (see Fig. 1). This observation confirmed pre-
vious results, which had led to the identification of the first species
as the chlorobenzene triplet and the latter one as the phenylperoxy
radical [12]. Under our conditions (low-energy pulses) no signifi-
cant ionization was observed (compare [32]). The exploration was
then extended to TFE, where the triplet was well apparent, indeed
with the same initial intensity as in cyclohexane, but was ca. 3
times shorter lived (see Fig. 2). Quenching by oxygen was likewise
observed, but the long-lived transient absorbance in the visible was
absent in this case.
There is no doubt that homolytic fragmentation of the C–Cl bond
(path a in Schemes 1 and 2) is the main reaction in cyclohexane as
indicated by the observed hydrogen abstraction (path d), as sup-
Furthermore, experiments were carried out in the presence
of ATMS in the range 0.01–0.05 M. Under these conditions, a
pseudo first-order decay was observed and from the experimen-
tal value kobs = kq × [ATMS] the rate of quenching of the triplet
was determined and found to be in the order of 108 mol−1 s−1
,
with a larger value in TFE than in cyclohexane (see Fig. 1,
Table 4).
4. Discussion
The observed behaviour for 1 in neat solvents can be explained
through the different pathways described in Scheme 2, the key
points of which are briefly indicated in the following. A first remark
Scheme 2. Photoreactions of 1 in neat solvent. The absorption maxima of the inter-
mediates revealed by flash photolysis are indicated.

S. Lazzaroni et al. / Journal of Photochemistry and Photobiology A: Chemistry 210 (2010) 140–144
143
solvent (43% 4 vs. 8% 3 yield, 0.075 ATMS/TFE molar ratio). This is
exactly what happens with bona fide triplet phenyl cation (e.g. as
obtained from the triplet sensitized decomposition of phenyldia-
zonium salts) [37] that smoothly add to ATMS (and to benzene) to
give arylated products as in the present case (paths h, i), while it
does not add to nucleophiles centered on a non-bonding orbital,
such as are alcohols. Notice that the yield of 4 is high only in the
presence of Et3N. This is again diagnostic and is due to the base
catalyzed elimination of the trimethylsilyl cation from adduct 9, a
phenomenon previously noted in related allyl and benzyl cations
[38,39]. In addition, since acidity is liberated during the reaction,
triethylamine preserve from degradation acid sensitive ATMS. As
one may expect, the base has no effect on the easy deprotonation
of Wheland adduct 10, nor on the desilylation of 9 in a nucleophilic
solvent such as MeOH.
Scheme 3. Possible excimer/exciplex path in the photochemistry of 1.
ported by the formation of bicyclohexyl and phenylcyclohexane,
as well as by oxygen trapping (path e). The latter reaction is known
to occur at a rate >1 × 1010 mol−1 s−1 [33] and leads to the per-
oxyl radical, identified by the absorption extended in the visible
[12,33,34]. The efficiency of homolytic cleavage depends on the
starting concentration of 1, with a significant decrease at 1 × 10−2 M
and above, known to be due to the formation of an excimer (6, see
Scheme 3) that is less reactive than the monomer excited state [10].
Et3N has only little effect with 1 as noticed in previous work [3]. ␲
donors, on the other hand, do affect the reaction. Thus, irradiation
of 1 in neat benzene had been previously found to give biphenyl
[35] and in the present work it has been found that phenylation
occurs also with an alkene, ATMS. These are minor reactions in
cyclohexane. By analogy with the above, these reactions might
be explained through the intermediacy of exciplexes with such
␲ donors (7 with benzene, see Scheme 3, and 8 with ATMS, see
Scheme 4, path f). Indeed, the data in Table 4 suggest that ca. 80%
of triplet chlorobenzene is quenched by ATMS under preparative
conditions (0.05 M). This leads to exciplexes that, contrary to the
case of 6, arise from different components and thus have an asym-
metric electronic distribution. This gives some chance to chemical
reaction as opposed to unproductive decay. What would be the
result of such interaction is doubtful, either reduction or alkyla-
tion. The former may be more likely, because there is abundant
evidence in the literature of reductive dechlorination of aryl halides
via an exciplex with a ␲ donor [36], but it is possible to explain the
formation of the small amount of allylbenzene found in cyclohex-
ane to path f. In principle, one may extend this rationalization and
attribute the larger yield of product 4 in polar media to the same
path, by taking into account the increased charge separation in the
exciplex.
As mentioned above, while solvolysis poorly competes with
addition to the alkene, product 3 is formed in a rather high yield in
the absence of ATMS. This fits with the proposed mechanism since
the cation is initially formed from triplet 31 and thus in the triplet
multiplicity, but ISC to the much more stable singlet then occurs
(path j, Scheme 2) [40]. Thus, in the presence of a good trap for
such species such as a ␲ nucleophile (reaction rate >109 M−1 s−1
)
[41], the cation is trapped as in the triplet state as soon as formed,
independently of whether it is formed via unimolecular cleavage
or via exciplex. In the contrary case, there is time for ISC to the sin-
glet cation, an unselective electrophile that mainly reacts with the
solvent (path k).
It should be further distinguished whether the phenyl cation
is the primary product from 31 (path c), or the photochemical
cleavage is in any case homolytic, but in polar solvents electron
transfer between the radical and the chlorine atom produces the
cation (path cꢀ in Scheme 2). Should the latter hypothesis apply,
the reaction would be in every case subject to quenching by oxy-
gen that efficiently traps phenyl radicals. However, flash photolysis
experiments show that no phenylperoxy radical is formed in TFE.
Furthermore, a comparison of the reactions in TFE/O2 in the pres-
ence and absence of ATMS (see Tables 1 and 2) evidences that
solvolysis is quenched, while alkene arylation is not. This supports
that in this solvent the latter process involves fast trapping either of
triplet 11 or of the first formed intermediate 3Ph+ by a ␲ trap, with
no oxygen interference. In contrast, the latter process requires ISC
to 1Ph+ and is more easily perturbed. At any rate, notice that path cꢀ
would give the cation in the singlet state, reacting indiscriminately
with n and ␲ nucleophiles, contrary to what observed.
Summing up the various pieces of evidence, the homolytic
fragmentation of the chlorobenzene triplet (31), characterized in
previous work [3,6,10–12], is identified by oxygen trapping of
phenyl radicals. Shifting to a polar solvent does not affect the sin-
glet (11), but shortens the lifetime of triplet. The reaction quantum
yield decreases under these condition, indicating a favored ISC
back to the ground state. Donor-assisted heterolytic cleavage has
some role, though, and gives the triplet phenyl cation, efficiently
quenched by ␲ nucleophiles, not by oxygen. More precisely, this
path appears to be favored by acidity rather than polarity (com-
pare acidic TFE, pKa = 12.37, with MeOH, pKa = 15.49, while the
latter solvent is more polar). Solvent co-operation to heterolytic
cleavage could involve some stabilization of the charge on the leav-
ing chloride. Hydrogen bonding to chlorobenzene is notoriously
scarce, but there is some indication of it in the ground state [42]
and it appears conceivable that an acidic alcohol has a serious
role in facilitating heterolysis from the excited state via a com-
plex Ph–Cl· · ·HOCH2CF3. The rate of non-assisted heterolysis under
these conditions is around 1 × 106 s−1 or somewhat below (see
Fig. 1), in the same region as the homolysis of 1 [32], but much
below heterolysis of electron-donating substituted chlorobenzenes
(>108 s−1) [38].
However, the changes occurring in going to polar media, or at
least in TFE, seem to us better rationalized through the interven-
tion of triplet phenyl cation from the heterolytic fragmentation of
31 (probably assisted by the donor, path fꢀ). In fact, arylated product
4 substitutes solvolysis product 3 in TFE and the data in Table 2 show
that ATMS traps the intermediate 70 times more efficiently than the
Scheme 4. Photoreactions of 1 in the presence of allyltrimethylsilane (ATMS) or
benzene as ␲ nucleophile.

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5. Conclusion
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Acknowledgement
Partial support of this work by MIUR, Rome is gratefully
acknowledged.
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